Compositions comprising phosphate esters and 2,6-diamino-pyridine as an antioxidant

ABSTRACT

COMPOSITIONS COMPRISING A MAJOR AMOUNT OF A PHOSPHATE ESTER BASE STOCK AND AN OXIDATION INHIBITING AMOUNT OF 2,6-DIAMINOPYRIDINE ARE USEFUL AS OXIDATION RESISTANT FUNCTIONAL FLUIDS, PARTICULARLY AS AIRCRAFT HYDRAULIC FLUIDS.

-United States Patent 3,783,132 COMPOSITIONS COMPRISING PHOSPHATE ESTERS AND 2,6-DIAMINO-PYRIDENE AS AN ANTIOXIDAN'I John F. Herbert, St. Louis, Mo., assignor to Monsanto Company, St. Louis, M0. N0 Drawing. Filed Mar. 24, 1972, Ser. No. 237,903 Int. Cl. C09k 3/00; C10m 3/40 U.S. Cl. 252-78 10 Claims ABSTRACT OF THE DISCLOSURE Compositions comprising a major amount of a phosphate ester base stock and an oxidation inhibiting amount of 2,6-diaminopyridine are useful as oxidation resistant functional fluids, particularly as aircraft hydraulic fluids.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to oxidation resistant functional fluid compositions and particularly to phosphate ester based functional fluids containing a minor amount of 2,6-diaminopyridine as an oxidation inhibitor. Such compositions are useful as lubricants and hydraulic fluids.

Description of the prior art Phosphate ester based fluid compositions are widely used as the operating fluid in aircraft hydraulic systems where the fluids are exposed to elevated temperatures in the order of 300 F. It is essential to the successful operation of such hydraulic systems that the fluid used therein be resistant to oxidation at the operating conditions.

Many hydraulic systems employ significant amounts of magnesium and copper and alloys thereof in pumps, seals and associated hardware which is contacted by the hydraulic fluid. Phosphate ester based hydraulic fluids which are not effectively stabilized against oxidation form corrosive decomposition products which are particularly aggressive toward magnesium and copper. In order to prevent corrosive damage to mechanical components of such hydraulic systems therefor, it is essential that the hydraulic fluid be suitably stabilized.

A number of oxidation inhibitors have been suggested by the art and many provide effective stabilization of phosphate esters. In particular, aromatic secondary amines such as p,p'-diocty1 diphenyl amine have been used with good results. There is however, a continuing search for other stabilizers which are equal to or better than the known stabilizers inhibiting oxidation and which may be used alone or in combination with such known inhibitors to provide improved or more economical products.

It is accordingly an object of the present invention to provide a novel, inhibitor for phosphate ester based bydraulic fluids. It is a further object of this invention to provide an oxidation inhibitor to reduce magnesium and copper corrosion caused by phosphate esters exposed to elevated operating temperatures. These and other objects will become more apparent from the ensuing description and claims.

SUMMARY The oxidation resistant compositions of the present invention comprise a major amount of a phosphate ester base stock and an oxidation inhibiting amount of a 2,6- diaminopyridine compound represented by the structure.

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DESCRIPTION OF PREFERRED EMBODIMENTS The compositions of this invention comprise a major amount of one or more phosphate ester represented by the formula wherein each R is individually an alkyl or alkoxyalkyl group containing from 2 to about 12 carbon atoms, a phenyl group, or a substituted phenyl group containing up to about 16 carbon atoms wherein said substituents are alkyl, phenyl, or phenylalkyl groups and the alkyl has from 1 to about 10 carbon atoms.

The term major amount as employed herein is defined to mean at least 50 percent by weight. The compositions of this invention preferably comprise at least about 65 percent by weight of the phosphate ester and at least about 0.001 percent by weight of the phosphate ester of 2,6-diaminopyridine inhibitor. Any remainder of the composition may be selected from other base stock constituents such as, for example, polyesters, polyphenyl ethers, polyphenyl thioethers, polyamides, silicate esters, and the like which may be present in amounts of up to about 35 percent by weight of the composition and from conventional functional fluid additives as hereinafter described.

An amount of the 2,6-diaminopyridine inhibitor sufi'icient to improve the oxidative stability of the fluid composition is employed. This amount may be from as little as 0.001 to as much as about 5 percent by weight of the phosphate ester in the composition. Amounts greater than 5 percent by weight can be employed if desired but no commensurate advantages are obtained thereby typically, from about 0.01 to 0.5 percent by weight of the inhibitor is effective to impart stability to the phosphate ester and is accordingly the preferred range of concentration.

Preferred phosphate esters which can be employed as base stocks in the compositions of the instant invention are those where the groups represented by R are alkyl, alkoxy-alkyl, phenyl, or substituted phenyl as defined above. These preferred base stocks are hereinafter referred to generically as phosphates and include trialkyl phosphates, triaryl phosphates, alkyl diaryl phosphates,

dialkyl aryl phosphates, and mixtures of such phosphates,

Representative alkyl groups include ethyl, propyl, isopropyl, butyl, hexyl, Z-ethylhexyl, dodecyl, decyl, and the like. In the case of trialkyl phosphates, it is preferred that the total number of carbon atoms be from 12 to 36. Preferred alkoxyalkyl groups are those having the formula wherein n is an integer having a value of from 1 to 10, a is an integer having a value of from 2 to 10, and x is an integer of from 1 to 10 and preferably x is 1. Representative alkoxyalkyl groups include methoxyethyl, ethoxyethyl, propoxyethyl, ethoxypropyl, ethoxypentyl, propoxydecyl, nonyloxyethyl, octyl, oxybutyl, and the like.

Alkyl-substituted phenyl groups are preferably those containing up to about 16 carbon atoms wherein the alkyl groups contain from 1 to about 10 carbon atoms provided that the total number of carbon atoms in all at most about 10 carbon atoms. Also included Within the definition of substituted phenyl groups are, for example, phenyl alkyl phenyl groups wherein the alkyl contains from 1 to about 10 carbon atoms such as, for example, cumylphenyl, phenylmethylphenyl, phenylethylphenyl, phenylpropylphenyl, phenyloctylphenyl, and the like. Also included are phenyl-substituted phenyl groups such as, mand p-phenylphenyl, o-, mand p-methylphenylphenyl, o-, mand p-dimethylphenylphenyl, and the like.

Specific examples of preferred phosphate esters include dibutylphenyl phosphate, triphenyl phosphate, tricresyl phosphate, tributyl phosphate, tri-2-ethylhexyl phosphate, trioctyl phosphate, and mixtures thereof, including particularly mixtures of tributyl phosphate and tricresyl phosphate, and mixtures of isooctyl diphenyl phosphate and Z-ethylhexyl diphenyl phosphate. Particularly preferred phosphate esters and mixtures of esters are those which remain liquid at temperatures of about 30 C.

The fluid compositions of this invention can also contain minor amounts of conventional additive materials such as dyes, pour point depressants, metal deactivators, acid scavengers, antioxidants other than 2,6-dlaminopyridine, defoamers, and viscosity index improvers.

Preferred polymeric viscosity index improvers which may be employed in the compositions of this invention are the polymers of alkyl esters of alpha-beta unsaturated monocarboxylic acids having the formula wherein R' and R" are each individually hydrogen or a C to C alkyl group and R" is a C to C alkyl group. Illustrative of the alkyl groups represented by R, R" and R" include, for example, methyl, ethyl, propyl, butyl, tbutyl, isopropyl, 2-ethylhexyl, hexyl, decyl, undecyl, dodecyl, and the like. Such polymers include, for example, polybutyl methacrylate, polyhexyl methacrylate, polyoctyl acrylate, polydodecyl acrylate, and compounds obtained by esterifying the alpha-beta unsaturated monocarboxylic acid with a mixture of monoalcohols containing from 1 to 12 carbon atoms.

Preferred V.I. improvers are the polyalkyl acrylates and polyalkyl methacrylates in which the alkyl groups have from about 4 to 12 carbon atoms and the molecular weight of the polymer is from about 5,000 to about 40,000. The

Epoxide compounds containing more than one epoxide group per molecule include, for example,

epoxidized soybean oil,

glyceryl,

tris acetoxyepoxy) stearate,

epoxidized linseed oil,

di( 2-methyl-4,S-epoxycyclohexylmethyl) adipate,

4,S-epoxycyclohexylmethyl(3,4-epoxycyclohexylmethyl) carboxylate,

3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate,

3,4-epoxy-1-methylcyclohexylmethyl-3,4-epoxy-l-methylcyclohexanecarboxylate,

6-methyl-3,4-epoxycyclohexylmethyl-6-methyl-3,4-epoxycyclohexanecarboxylate,

3,4-epoxy-2-methylcyclohexylmethyl-3,4-ep0xy-2- methylcyclohexanecarboxylate,

vinylcyclohexane dioxide,

butadiene dioxide,

1,4-bis 2,3-epoxypropoxy) benzene,

1,3-bis(2,3-epoxypropoxy)benzene,

l,3-bis(2,3-epoxybutoxy)benzene,

4,4 '-bis(2,3-epoxypropoxy)diphenyldimethylmethane,

1,3-bis 2-hydroxy-3,4-epoxybutoxy) benzene,

the polymer prepared by reacting sorbitol with epichlorohydrin, poly(allyl-2,3-epoxypropyl ether), poly(2,3-epoxypropyl crotonate), and the like.

The oxldative stability of the composition of the instant invention were determined according to the basic procedure set forth in Federal Test Method Standard 791- 5 308 under specific test conditions as follows:

Sample size-100 cc.

Airfiow5 liters/hr. of dry air T emperature-300 F.

Exposure time-168 hours Metals present-Mg, Al, Cd, Fe, Cu

The effect of incorporating 0.1 percent of 2,6-diaminopyridine in one base stock consisting of dibutyl phenyl phosphate and in a second base stock consisting of a mixture of tributyl phosphate and tricresyl phosphate is illustrated by the Oxidation and Corrosion Test results given in Table I. It is evident from this data that in both instances the addition of the inhibitor of this invention elfectively inhibited the corrosion rate experienced during the test, particularly with respect to magnesium and copper. There was also significant improvement in reducing the amount by which viscosity and acidity increased during the test.

TABLE L-OXIDATION AND CORROSION TEST 100 F. Corrosion rate, mgJcm. viscosity, Acidity, TAN

ercent Fluid composition Mg Al Cd Fe Cu Coking Sludge increase Initial Final A. Dibutyl phenyl phosphate 60.85 -0.04 82.09 -2. 75 l87.94 Light... Heav 514 0.02 B. (A) /plus 0.1% 2,6-diaminopyridine 0. 23 0.06 0.03 +0. 06 12.52 Heavy. do.i.- 4.0 0.02 C. 0 tributyl phosphate; 11.5% tncresyl phos- 0. 14 0.00 6. 89 +0.04 5l.04 None...- None...- 5.7 0.02 10. 1

p a e.

D. (0) plus 0.1% 2,6-diaminopyridine 0.06 -0.03 0.05 +0.06 +0.04 Heavy-..-.do.-.-- 0 0. 02 1.8

Applicant has discovered that it is only the 2,6-diaminopyridine isomer which is able to inhibit the oxidative degradation of phosphate esters. Tests conducted with 2,3- diaminopyridine and 2,5-diaminopyridine revealed that these isomers did not improve the performance of base stock material and in some instances had an adverse effect thereon. Likewise, the mono-substituted aminopyridines were discovered to be ineifective as oxidation inhibitors for the compositions of this invention. In view of the negative results obtained with the aminopyridines and with the other isomers of diaminopyridine, it is most surprising to find the 2,6-diaminopyridine isomer to be such a remarkably effective inhibitor as illustrated by the data in Table I.

Although 2,6-diaminopyridine is the preferred additive of this invention, alkyl and aryl substituted derivatives thereof are also encompassed within the scope of the invention. For example, 2,6-diaminopyridine may be substituted with one or two alkyl radicals such as methyl, ethyl, propyl, butyl, hexyl, or octyl. Alternatively, the substituents may be aryl radicals such as phenyl, tolyl, or xylyl. Such substituted 2,6-diaminopyr'idine compounds are also effective oxidation inhibitors for phosphate ester based functional fluids.

While this invention has been described with respect to various specific examples and embodiments, it is to be understood that the invention is not limited thereto. Specifically, the compositions of this invention comprise a phosphate ester base, stock and 2,6-diaminopyridine as the critical ingredients, but many other ingredients can be incorporated within the composition, including for example, other base stocks and other additive materials. The incorporation of such other ingredients may materially affect some properties of the resulting composition, as for example viscosity, viscosity index, lubricity, foamability, flammability, and the like but in no way diminish the utility of the instant invention. It is apparent therefore that the compositions of the instant invention are not to be limited to the essential ingredients as defined herein, but may be used in combinations and mixtures with numerous other compositions as would be readily apparent to those skilled in the art.

The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows:

1. A composition useful as a hydraulic fluid comprising (A) at least 50% by weight of a phosphate ester or mixture of phosphate esters represented by the formula wherein each R is individually an alkyl or alkoxyalkyl group containing from 2 to about 12 carbon atoms, a phenyl group, or a substituted phenyl group containing up to about 16 carbon atoms wherein the substituents are alkyl, phenyl, or phenylalkyl groups, and (B) an oxidation inhibiting amount of a 2,6-diaminopyridine compound represented by the structure R)ll wherein each R is individually an alkyl of from 1 to 8 carbon atoms, a phenyl or a methyl substituted phenyl, and n is an integer of 0 to 2.

2. A composition of claim 1 wherein the amount of phosphate ester is at least about percent by weight of the total composition and the amount of the 2,6-diaminopyridine compound is at least about 0.001 percent by weight of the phosphate ester.

3. A composition of claim 2 wherein the phosphate ester is selected from the group consisting of trialkyl phosphate and triaryl phosphate, alkyl aryl phosphate, dialkyl aryl phosphate, and mixtures of such phosphates.

4. A composition of claim 3 wherein the phosphate ester is dibutyl phenyl phosphate.

5. A composition of claim 4 wherein n is zero and the amount of 2,6-diaminopyridine is from about 0.01 to 0.5 percent by weight of the phosphate ester.

6. A composition of claim 3 wherein the phosphate ester is a mixture of tributyl phosphate and tricresyl phosphate.

7. A comopsition of claim 6 wherein n is zero and the amount of 2,6-diaminopyridine is from about 0.01 to 0.5 percent by weight of the phosphate ester.

8. A composition of claim 2 additionally containing from about 2 to about 20 percent by weight of a viscosity index improver which is a polymer of an ester having the structure 0 R' lJ- oR'" H wherein R and R" are each individually hydrogen or a C to C alkyl group and R' is a C to C alkyl group. 9. A composition of claim 2 additionally containing from about 0.01 to 5 percent by weight of an acid scavenger which is a monoor polyepoxide compound.

10. A composition of claim 9 wherein the acid scavenger is an epoxycyclohexanecarboxylate.

References Cited UNITED STATES PATENTS 2,318,196 5/1943 Chenicek 252-401 X 2,549,270 4/1951 Watson 252-78 3,226,324 12/ 1965 Eickermeyer 25250 X 3,159,582 12/1964 Sims 252----153 3,226,324 12/1965 Eickemeyer 252-50 X 3,468,802 9/1969 Nail 252-78 3,591,500 7/1971 Sullivan 25278 X 3,637,507 l/1972 Gentit 25278 LEON D. ROSDOL, Primary Examiner H. A. PITLICK, Assistant Examiner US. Cl. X.R.

2 3 UNITED STATES PATENT OFFICE QERTIFICATE OF CORRECTION Patent No- 3'783'l32 Dated January 1, 1974 Inventofla) John" F Herber It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 5; "John F. Herbert" should be John F. Herber Signed and sealed this 14th day of June 197L|..

(SEAL) Attest:

EDWARD M.FLETCHER,J'R. C. MARSHALL DANN Commissioner of Patents Attesting Officer 2 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,783,132 1m January 1, 1974 Inventor(s) John F. Herber It is certified that error appear: in the above-identifid patent and thlt said Letters Patent are horaby correctod as shown below:

Column 1, line 5; "John F. Herbert" should be John F. Herber Signed and saaled th1s.hth day of Juno l97h.

(SEAL) Attest:

O. MARSHALL DANN Commissioner of Patents EDWARD M.FLETCHER,JR. Attesting Officer 

